1. Field of the Invention
The present invention relates to a semiconductor device, e.g., a photosensitive device with internal circuitry that includes on the same substrate both a photosensitive device for converting incident light into an electrical signal and an integrated circuit portion for processing the electrical signal which is output from the photosensitive device, and in particular to a high-performance semiconductor device incorporating a photosensitive device having an enhanced response speed; and a method for producing the same.
2. Description of the Related Art
Photosensitive devices with internal circuitry, which are composed of semiconductor devices, are employed for optical pickups, for example. In recent years, optical pickups employed in CD-ROM, CD-R/RW, or DVD-ROM drives have been increasing in operation speed, and there has been a demand for higher-performance (i.e., higher sensitivity and lower noise) photosensitive device having internal circuitry.
For example, obtaining a photosensitive device with internal circuitry having a high response speed requires at least a photodiode having rapid photoelectric conversion characteristics. In an attempt to enhance the photoelectric conversion characteristics of a photodiode, a semiconductor device shown in FIG. 9 has been proposed (Japanese Laid-Open Patent Publication No. 10-209411), for example. The semiconductor device shown in FIG. 9 includes bipolar transistors and a photodiode structural portion on a P-type semiconductor substrate 501. The photodiode structural portion includes a photodiode of a cathode-common type and a photodiode of an anode-common type. The following description will be focused on the photodiode of an anode-common type shown in FIG. 9.
A high concentration P-type embedded diffusion layer 502, an ultra-low concentration P-type epitaxial layer 503, and an N-type epitaxial layer 507 are laminated in this order on a P-type semiconductor substrate 501. In the P-type epitaxial layer 503, a P-type separation diffusion layer 504 is provided which extends from the surface of the P-type epitaxial layer 503 into the P-type embedded diffusion layer 502. A P-type embedded diffusion layer 506 is provided in the N-type epitaxial layer 507 so as to overlie the P-type separation diffusion layer 504. In accordance with this structure, split photodiodes of an anode-common type (i.e., which share the same anode portion in common), each having a rectangular shape, can be formed as shown in FIGS. 10A to 10C, for example. Herein, the P-type separation diffusion layer 504 and the P-type separation diffusion layer 506 together compose a partitioning portion which splits the four rectangular photodiode regions from one another. In order to improve the frequency characteristics of the photodiodes, it is necessary to reduce the junction capacitance and serial resistance in the first place.
In accordance with the above-described structure, the junction capacitance of concern exists between the P-type epitaxial layer 503 and the N-type epitaxial layer 507. Since the P-type epitaxial layer 503 has an ultra-low concentration, it is possible to ensure a sufficient expanse of a depletion layer from the N-type epitaxial layer 507 to the P-type epitaxial layer 503 when a reverse voltage is applied. As a result, the junction capacitance between the P-type epitaxial layer 503 and the N-type epitaxial layer 507 can be reduced. The serial resistance of concern is determined by the serial resistance of the high concentration P-type embedded diffusion layer 502 and the P-type embedded diffusion layer 504. Since both layers 502 and 504 have a high concentration and a small resistivity, their serial resistance can also be made small. The frequency characteristics of the split photodiodes can be improved in this manner.
An optical pickup is operative to track the data carried on a disk which is rotating at a fast rate, so as to read out a reproduction signal therefrom, while acquiring servo signals which are provided for facilitating the accurate reading of the data from the disk. The servo signals include a focus error signal (FES) for positioning the focal point of laser light emitted from a semiconductor laser on the disk and a radial error signal (RES) for positioning the focal point of the laser light on a certain pit (or track) on the disk. The latter positioning control is often referred to as xe2x80x9ctrackingxe2x80x9d. A number of methods exist for detecting such servo signals. As an example of a method for detecting an FES, an astigmatic method will be described below.
In order to detect an FES by the astigmatic method, it is necessary to employ four split photodiodes which have respectively different light-receiving regions. FIGS. 10A to 10C illustrate how a beam spot may appear on a photosensitive device in accordance with the astigmatic method. FIG. 10A shows the case where a focal point of laser light emitted from a semiconductor laser is on the surface of a disk, in which case the beam spot has a truly circular shape. FIGS. 10B and 10C show the cases where a focal point of laser light emitted from a semiconductor laser is in front of or behind the surface of a disk, respectively, in which cases the beam spot has an elliptical shape. The different shapes of light beams are ascribable to the use of a cylindrical lens which exerts a lens effect on only light which is polarized in a certain direction. An FES can be obtained by applying the respective output signals Sa, Sb, Sc, and Sd from the four split photodiodes PDa, PDb, PDc, and PDd (as shown in FIGS. 10A to 10C) to the following equation:
FES=(Sa+Sd)xe2x88x92(Sb+Sc),
where a difference between a sum of the output signals from one pair of diagonally-disposed photodiodes and a sum of the output signals from the other pair of diagonally-disposed photodiodes is derived. Different calculation results of this equation correspond to different beam spot convergence states as follows:
FIG. 10A (where the focal point of laser light is on the disk surface): FES=0
FIG. 10B (where the focal point of laser light is in front of the disk surface): FES greater than 0
FIG. 10C (where the focal point of laser light is behind the disk): FES less than 0
Accordingly, by performing a feedback control so that the value of FES equals zero, the beam spot will be properly placed on the disk surface.
The state shown in FIG. 10A is also a state where a reproduction signal RF is constantly being read based on proper servo control. Hence, the reproduction signal RF can be calculated by taking a sum of the output signals Sa, Sb, Sc, and Sd from the four split photodiodes PDa, PDb, PDc, and PDd, as shown by the following equation:
xe2x80x83RF=Sa+Sb+Sc+Sd.
In view of the aforementioned manner of using an optical pickup, it can be seen that enhancing the performance of an optical pickup in speed, sensitivity, and noise level can only be achieved through enhancing its performance in the state where laser light is incident on the partitioning portion (i.e., the P-type separation diffusion layer 504 and the P-type separation diffusion layer 506) as well as the light-sensitive portions of the split photodiodes. However, the response of the split photodiodes of an anode-common type as shown in FIG. 9 generally deteriorates when laser light is incident on the partitioning portion for the following reasons.
Laser light of a wavelength of 780 nm or 650 nm (which are typically employed for an optical pickup) would intrude into the partitioning portion of an optical pickup to a depth of about 9 xcexcm or about 3.5 xcexcm, respectively. The xe2x80x9cdepthxe2x80x9d as used herein is defined as a depth at which the light intensity is reduced to 1/e that of the incident light (where e represents the base of a natural logarithm). Therefore, when laser light is incident on the partitioning portion, a large portion of photocarriers are generated in the high concentration P-type separation diffusion layer 504 or in the underlying P-type embedded diffusion layer 502. The generated photocarriers will migrate within the high concentration separation diffusion layer 502 through diffusion as shown in FIG. 11. Now, the partitioning portion needs to have a width of about 2 xcexcm or more, for example, in order to secure a certain level of separation withstand pressure: in such cases, a distance of about 1 xcexcm or more will be traveled by photocarriers via diffusion. The photocarrier migration via diffusion occurs more slowly than the migration which occurs via an electric field generated across the depletion layer (called the xe2x80x9cdepletion layer fieldxe2x80x9d). On the other hand, when laser light is radiated in regions other than the partitioning portion of the split photodiodes, a large portion of the generated photocarriers will rapidly migrate via a depletion layer field. Thus, when light is incident on the partitioning portion of a photodiode, a signal component emerges which has a relatively slow response speed, so that the response speed will decrease as compared with the case where light is not incident on the partitioning portion.
According to one aspect of the present invention, there is provided a semiconductor device including: a semiconductor multilayer structure of a first conductivity type; a first semiconductor layer of a second conductivity type, formed on the semiconductor multilayer structure of the first conductivity type; a photosensitive section composed essentially of a PN junction between the semiconductor multilayer structure of the first conductivity type and the first semiconductor layer of the second conductivity type; and a partitioning portion for splitting the photosensitive section into a plurality of regions, wherein the semiconductor multilayer structure of the first conductivity type includes: a semiconductor substrate of the first conductivity type; a first semiconductor layer of the first conductivity type formed on the semiconductor substrate of the first conductivity type; and a second semiconductor layer of the first conductivity type formed on the first semiconductor layer of the first conductivity type, wherein the photosensitive section is formed in a region surrounded by a third semiconductor layer of the first conductivity type, the third semiconductor layer of the first conductivity type extending from a surface of the first semiconductor layer of the second conductivity type so as to reach the second semiconductor layer of the first conductivity type, and a fourth semiconductor layer of the first conductivity type is provided under the third semiconductor layer of the first conductivity type, wherein the fourth semiconductor layer of the first conductivity type overlaps with at least a portion of the third semiconductor layer of the first conductivity type, extends through the second semiconductor layer of the first conductivity type, and at least reaches the first semiconductor layer of the first conductivity type, and wherein the partitioning portion includes a fifth semiconductor layer of the first conductivity type extending from the first semiconductor layer of the second conductivity type so as to reach the second semiconductor layer of the first conductivity type but not to reach the first semiconductor layer of the first conductivity type.
Thus, when light is incident on the partitioning portion of the photosensitive device, photocarriers generated in a portion which is closer to the surface than a diffusion profile peak ascribable to the first semiconductor layer of the first conductivity type are accelerated by an internal field, thereby enhancing the response speed of the photodiode. Furthermore, the photocarriers generated in the portion which is closer to the surface than the diffusion profile peak ascribable to the first semiconductor layer of the first conductivity type are blocked by a potential barrier, so that the slow-responding photocarriers which are generated in any deeper portion are prevented from contributing to the response of the device. As a result, the response speed of the entire photosensitive device is enhanced.
In one embodiment of the invention, the fourth semiconductor layer of the first conductivity type has an impurity concentration of about 1xc3x971014 cmxe2x88x923 or more in a portion contacting the first semiconductor layer of the first conductivity type.
In accordance with the aforementioned structure, the serial resistance of the photodiode can be sufficiently reduced, whereby the response speed of the photodiodes can be enhanced.
In another embodiment of the invention, the fourth semiconductor layer of the first conductivity type has an impurity concentration of about 1xc3x971018 cmxe2x88x923 or less at an interface with the third semiconductor layer of the first conductivity type.
In accordance with the aforementioned structure, autodoping layers are prevented from being formed within the semiconductor device at the time of forming the first semiconductor layer of the second conductivity type. Thus, the response speed of the photodiodes can be prevented from deteriorating.
In still another embodiment of the invention, a plurality of electrode leads having one of opposite polarities of the photosensitive section are provided in a periphery of the plurality of split regions of the photosensitive section.
In accordance with the aforementioned structure, even if the serial resistance of each element is relatively high, a plurality of such elements (having identical resistance) in a parallel arrangement serves to reduce the overall serial resistance, whereby the response speed of the photodiodes can be enhanced.
In still another embodiment of the invention, the first semiconductor layer of the first conductivity type has a higher impurity concentration than an impurity concentration of the semiconductor substrate of the first conductivity type.
In still another embodiment of the invention, the second semiconductor layer of the first conductivity type has a lower impurity concentration than an impurity concentration of the first semiconductor layer of the first conductivity type.
In still another embodiment of the invention, an impurity concentration distribution in the first semiconductor layer of the first conductivity type has a gradient.
In still another embodiment of the invention, the semiconductor device further includes a transistor formed on the semiconductor multilayer structure of the first conductivity type.
In accordance with the aforementioned structure, both a photoelectric conversion portion and a processing portion for processing an electrical signal obtained through photoelectric conversion can be integrated on the same substrate.
According to another aspect of the present invention, there is provided a method for producing a semiconductor device including a photosensitive section, the method including the steps of: forming a first semiconductor layer of a first conductivity type on a semiconductor substrate of the first conductivity type, the first semiconductor layer of the first conductivity type having a higher impurity concentration than an impurity concentration of the semiconductor substrate of the first conductivity type: forming a second semiconductor layer of the first conductivity type on the first semiconductor layer of the first conductivity type, the second semiconductor layer of the first conductivity type having a lower impurity concentration than an impurity concentration of the first semiconductor layer of the first conductivity type; selectively forming an embedded semiconductor layer of a second conductivity type on the second semiconductor layer of the first conductivity type via a first thermal process; selectively forming a fourth semiconductor layer of the first conductivity type via a second thermal process so as to extend through the second semiconductor layer of the first conductivity type and to at least reach the first semiconductor layer of the first conductivity type; forming a first semiconductor layer of the second conductivity type on the second semiconductor layer of the first conductivity type; forming a third semiconductor layer of the first conductivity type so as to extend from the first semiconductor layer of the second conductivity type and to reach the fourth semiconductor layer of the first conductivity type; and forming a fifth semiconductor layer of the first conductivity type so as to extend from the first semiconductor layer of the second conductivity type and to reach the second semiconductor layer of the first conductivity type.
In one embodiment of the invention, the first thermal process and the second thermal process are performed substantially simultaneously.
As a result, the number of production steps, and hence the production cost, can be reduced.
In another embodiment of the invention, the fifth semiconductor layer of the first conductivity type is formed so as not to reach the first semiconductor layer of the first conductivity type.
Thus, the invention described herein makes possible the advantages of (1) providing a photosensitive device with internal circuitry that includes on the same substrate both a photodiode portion and an integrated circuit portion, such that the photodiode portion composes fast-operating split photodiodes whose response speed is not decreased when laser light is incident on the partitioning portion of the split photodiodes; and (2) providing a method for producing the same.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.